The present invention relates to a fuel injection control system for a diesel engine, and more particularly to a system arranged to execute a pilot fuel injection prior to a main fuel injection.
Such a pilot fuel injection is effective to reduce vibrations due to combustion of a diesel engine and is executed when an engine operating condition is in a predetermined operating zone where the combustion vibration of the diesel engine becomes large. Japanese Patent Provisional Publication No. 61-272450 discloses a typical fuel injection control system arranged to execute a pilot injection in a predetermined engine operation zone.
However, the demand for the pilot fuel injection depends on various factors of an operating condition and internal and external circumstances of the engine. For example, the degree of the demand for the pilot fuel injection varies according to whether the engine is put in a stationary state or transient state. Therefore, it is difficult to appropriately execute the pilot fuel injection in the predetermined engine operation zone.
It is therefore an object of the present invention to provide an improved fuel injection system which is arranged to provide a plurality of membership functions indicative of degree of the demand for executing the pilot fuel injection with respect to the parameters indicative of the engine operating condition as evaluation functions and to decide the execution of the pilot fuel injection on the basis of the evaluation functions so that the decision as to whether the pilot fuel injection should be executed is finely executed even when there is a difference between a stationary state and a transient state or a dispersion in circumstances.
Another object of the present invention is to provide an improved fuel injection system which enables the pilot fuel injection and the main fuel injection to be optimally controlled in a low-load region of the engine operating condition.
A fuel injection control system according to the present invention is for a multi-cylinder diesel engine and comprises a control unit. The control unit is arranged to decide whether a pilot fuel injection is executed, to decide whether an operating condition of the engine is in a predetermined low-load operating region, to calculate a mono-cylinder fuel injection quantity for each cylinder of the engine when the engine operating condition is in the predetermined low-load operating region, to calculate a common fuel injection quantity common among the cylinders of the engine when the engine operating condition is out of the predetermined low-load operating region, to calculate a mono-cylinder pilot fuel injection quantity for each cylinder by multiplying a predetermined ratio to the fuel injection quantity when the engine operating condition is in the predetermined low-load operating region and when the pilot fuel injection is executed, to calculate a common pilot fuel injection quantity common among the cylinders when the engine operating condition is out of the predetermined low-load operating region and when the pilot fuel injection is executed, and to calculate a main fuel injection quantity by subtracting the pilot fuel injection quantity from the fuel injection quantity.